The present invention is directed, in general, to driver circuits for light emitting diode illumination sources and, more specifically, to voltage-controlled dimming driver circuits for light emitting diode illumination sources capable of providing illumination despite failure of one or more light emitting diodes within the illumination source.
Commercial and military aircraft instrumentation displays, like many other display systems, frequently employ illuminated indicators and controls. Traditionally, incandescent lamps operating at 5 VAC, 14 VDC or 28 VDC have been employed as illumination sources for illuminated pushbutton switches, indicators and annunciators within aircraft instrumentation. The illumination from such incandescent lamps is generally optically filtered to produce a wide range of human visible or night vision imaging system (NVIS) colors, and the small size of incandescent lamps allows multiple lamps to be used within the same display to illuminate different regions of the display in different colors.
The inherent characteristics of incandescent lamps, however, lead to noticeable chromaticity shifts as the applied voltage is reduced to adjust the output luminance from sunlight readability in daytime flying conditions to low luminance levels required for night flying conditions. Moreover, incandescent lamps suffer other disadvantages when employed in aircraft instrumentation, including high power consumption, high inrush current, uncomfortably high touch temperatures, and unreliability in high vibration environments. As a result, considerable effort has been expended to incorporate more stable, efficient and reliable technologies, such as light emitting diodes (LEDs), into aircraft crewstation illuminated displays. Light emitting diodes offer high luminous efficiency, low heat, low power consumption and high reliability.
The luminance required for aircraft instrumentation displays ranges up to approximately 400-500 foot-lamberts for sunlight-readability in daytime flying. In order to provide sufficient illumination to be visible in direct sunlight, multiple light emitting diodes may be required. For example, FIGS. 3A and 3B depict circuit diagrams for driver configurations which might be employed in light emitting diode illumination sources for aircraft instrumentation. Drivers 300 and 301 each include a biasing resistor R2 and a set of light emitting diodes L1-L4 connected in series between input and output ports (xe2x80x9c+xe2x80x9d and xe2x80x9cxe2x88x92xe2x80x9d) to which the input voltage is applied. In driver 300, the light emitting diodes L1-L4 are connected in parallel. Unfortunately, the power consumption of parallel light emitting diodes increases linearly with each additional light emitting diode, and can even consume as much power as the incandescent lamps which the light emitting diode illumination sources are designed to replace.
By connecting the light emitting diodes L1-L4 in series, as in driver 301, maximum power efficiency can be realized. Since current is constant in series-connected light emitting diodes, the power consumed by a display containing multiple series-connected light emitting diodes is no more than that consumed by a display containing only a single light emitting diode, while the total luminance increases with each light emitting diode added.
In a series-connected configuration such as driver 301, however, if any of light emitting diodes L1-L4 fails in an electrically open state, the entire string of light emitting diodes is rendered nonfunctional.
There is, therefore, a need in the art for a light emitting diode illumination source providing both power efficiency and fault tolerance for failure of one or more light emitting diodes in an electrically open condition.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a light emitting diode illumination source, a fault tolerance for failure of one light emitting diode within a series-connected group. In the present invention, such fault tolerance is provided by a zener diode connected in parallel with sets of series-connected light emitting diodes with a reverse forward bias orientation. The threshold voltage of the zener diode is matched to equal, or be just slightly greater than, the forward voltage drops for the light emitting diodes at maximum current. During normal operation, therefore, the zener diode draws virtually no current. Upon failure of one of the light emitting diodes, the zener diode conducts without increase in the applied voltage, providing an alternate current path maintaining circuit integrity so that other sets of light emitting diodes connected in series within the circuit continue to illuminate.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9corxe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases.